This invention relates to solidification processing of metallic alloys, and, more particularly, to semi-solid processing of metallic alloys.
The casting of a metal into a useful shape involves heating the metal to a temperature above its melting point, placing the molten metal into a form termed a mold, and cooling the metal to a temperature below its melting point. The metal solidifies in the shape defined by the mold, and is thereafter removed from the mold. Within these general guidelines, a wide variety of casting technologies are known.
When most metallic alloys are cooled from the molten state, they do not solidify at a single temperature, but over a temperature range. As the metal is cooled, it first reaches a liquidus temperature at which the alloy begins to freeze. As the temperature is further reduced, an increasing fraction of the metal becomes solid, until the metal is entirely solid below a solidus temperature.
In conventional casting practice, the metal is cooled from the molten state above the liquidus temperature to the solid state below the solidus temperature, without being held at a temperature between the liquidus temperature and the solidus temperature. However, it is known to cool the metal to a semi-solid temperature range between the liquidus temperature and the solidus temperature and hold the metal at that temperature, so that the metal is in a semi-solid state. Alternatively, the metal may be heated from a temperature below the solidus temperature to the semi-solid temperature range between the liquidus temperature and the solidus temperature. By whatever path the metal reaches this semi-solid temperature range, the semi-solid material is then processed to produce a structure of solid globules in a liquid matrix. This process may involve intensive stirring, but if suitable conditions are achieved to give many crystallization nuclei (for example by rapid cooling or using suitable grain refinement techniques) the process may involve only an aging step. The semi-solid mixture is then forced into a mold while in this semi-solid state, typically by die casting.
In the conventional semi-solid casting technique, careful control is required over the heating and cooling parameters, specifically the holding temperature at which the processing apparatus is maintained. The present inventors have realized that for commercial purposes the conventional approach is confined to use with alloys having a low rate of increase of the fraction of solids with decreasing temperature, at the semi-solid processing temperature. Consequently, many alloys are excluded from practical commercial semi-solid processing, unless a high degree of control on temperature (requiring expensive equipment) is achieved. This high degree of control is not possible or not practical for many commercial semi-solid casting operations
Accordingly, there is a need for an improved approach to the semi-solid casting of metallic alloys, which is less restrictive on processing parameters and produces a better-quality final product. The present invention fulfills this need, and further provides related advantages.
This invention provides a method for semi-solid processing of metallic alloys, which is operable with a variety of metals having both high and low variation of solids content with temperature in the semi-solid temperature range. The approach of the invention does not require intensive stirring and/or mixing in the semi-solid range, resulting in improved quality of the final cast product as a result of reduced incorporation of defects into the semi-solid material and thence into the cast product. The approach also allows the relative fraction of solid and liquid to be controllably varied in the semi-solid structure without changing temperature, so that the structure of the as-cast product may similarly be varied. Recycling of materials in the casting plant is also facilitated. In a preferred embodiment, temperature control of the metallic alloy is significantly simplified, with the result that materials having very narrow operable temperature ranges in the semi-solid state may be processed.
In accordance with the invention, a metallic alloy having a liquidus temperature and a solidus temperature is processed. The method comprises the steps of providing the metallic alloy having a semi-solid range between the liquidus temperature and the solidus temperature of the metallic alloy, heating the metallic alloy to an alloy initial elevated temperature above the liquidus temperature, reducing the temperature of the metallic alloy from the initial metallic alloy elevated temperature to a semi-solid temperature of less than the liquidus temperature and more than the solidus temperature, and maintaining the metallic alloy at the semi-solid temperature for a sufficient time to produce a semi-solid structure in the metallic alloy of a globular solid phase dispersed in a liquid phase, which is usually between 1 second and 5 minutes. The method optionally further includes removing at least some, but not all, of the liquid phase present in the semi-solid structure of the metallic alloy to form a solid-enriched semi-solid structure of the metallic alloy. The metallic alloy having the semi-solid structure or the solid-enriched semi-solid structure is formed into a shape.
In a particularly preferred embodiment, the metallic alloy is cooled from above the liquidus temperature to the semi-solid temperature by providing a crucible at a crucible initial temperature below the solidus temperature, pouring the metallic alloy into the crucible, and allowing the temperature of the metallic alloy and the crucible to reach an equilibrium at the semi-solid temperature. The relative masses and properties of the metallic alloy and the crucible and their initial temperatures are preferably selected such that, when thermal equilibrium between the two is reached, the metallic alloy and the crucible are at the desired semi-solid temperature. Temperature control is simplified, and metallic alloys with a high rate of weight fraction solids formation with decreasing temperature may be processed.
If the particularly preferred embodiment is used, the semi-solid mixture may be directly transferred to a die casting machine without solidifying it, and die casting the resulting semi-solid globularized mixture. However, it is preferred to include the step of removing at least some liquid phase prior to casting, as this permits the globularization step to occur under conditions where there is substantial liquid phase present, resulting in more efficient heat and mass transfer.
The removal of liquid phase, where used, is preferably accomplished by allowing liquid to drain from the semi-solid material through a filter or other porous structure, thereby increasing the relative amount of the solid material in the semi-solid material. In a typical case, the semi-solid structure initially has less than about 50 weight percent solid phase, preferably from about 20 to about 35 weight percent, and the liquid phase is removed until the solid-enriched semi-solid structure has from about 35 to about 55 weight percent, preferably about 45 weight percent, of solid phase present as determined by the procedures described subsequently.
After concentration of the solid weight fraction accomplished by removal of liquid phase, the metallic alloy is thixotropic. That it, it may be handled in the manner of a solid, but may then be formed to a final shape by any operable liquids-processing technique such as pressure die casting.
The present invention may be used with any material having a semi-solid range, but is preferably practiced with aluminum alloys. It may be performed with alloys that are reinforced with a phase that remains solid throughout processing, producing a final cast reinforced composite material.
The present invention also provides a modified alloy composition that is suitable for use with the processing described above. The modified alloy composition allows the production of solid product of a desired final composition when processed by the procedure in which some liquid phase is removed. In accordance with this aspect of the invention, a modified alloy composition comprises a base alloy having its solute elements adjusted to account for removal of a portion of the base alloy as a liquid phase at a semi-solid temperature between a liquidus temperature and a solidus temperature of the modified alloy composition, whereupon the remaining material after removal of the liquid phase has the base alloy composition. Stated alternatively, the invention provides a modified alloy whose composition is determined by the steps of providing a base alloy having a base alloy composition, and performing a separation procedure with the base alloy as a starting material. The separation procedure includes the steps of heating the starting material to above its liquidus temperature, cooling the starting material to a semi-solid temperature between its liquidus temperature and its solidus temperature, at which semi-solid temperature the starting material has a liquid portion and a solid portion of different composition than the liquid portion, and removing at least part of the liquid portion to leave a remaining portion having a remaining composition different from that of the starting material. A modified alloy composition is determined such that, when the modified alloy composition is processed by the separation procedure using the modified alloy as the starting material, its remaining composition is substantially the base alloy composition.
In conceiving the present invention, the present inventors have realized that, as a practical matter, the conventional approach to semi-solid processing is limited in a commercial setting to alloys having an absolute value of the temperature rate of change of percent solids at the holding temperature of about 1 weight percent solids per degree Centigrade or less. The present approach allows the semi-solid processing of alloys having an absolute value of the temperature rate of change of percent solids at the holding temperature that is greater than about 1 weight percent solids per degree Centigrade, and even greater than about 2 weight percent solids per degree Centigrade. The present approach therefore opens the way to the semi-solid processing of many alloys heretofore extremely difficult or impossible to process commercially. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.